An operation for hoisting and moving a heavy object such as a concrete block, etc. in a construction or building site has been conventionally frequently performed manually, i.e. by operators. However, various operation machines for clamping and hoisting a heavy object have been developed for saving time and labor and for preventing danger.
Many of these hoisting and clamping apparatuses are structured in that presser plates are brought into contact with both sides of a heavy object to thereby clamp the heavy object owing to friction generated between the heavy object and the presser plates. Such a hoisting and clamping apparatus is hoisted by a crane, etc. and it is moved to an intended spot while it clamps the heavy object. In the operation for clamping the heavy object by the hoisting and clamping apparatus, an operator is disposed in a position close to the heavy object in addition to an operator of the crane and the engagement or retention between the hoisting and clamping apparatus and heavy object must be set. Further, in installing the heavy object hoisted by the hoisting and clamping apparatus at an intended spot, to release the engagement or retention between the hoisting and clamping apparatus and heavy object, another operator must release such engagement. Under the circumstances, when the heavy object is hoisted and moved by this apparatus, the operator for operating the crane and another operator for setting and releasing the clamping of the heavy object at a position close to the heavy object are needed thereby costing personal expenses. Still further, another operator must work at a position close to the heavy object hoisted by the hoisting and clamping apparatus, which causes another operator to be involved in danger because there is a possibility that the heavy object falls due to the improper clamping of the heavy object.
As mentioned above, since the engagement or retention between the hoisting and clamping apparatus and heavy object are performed manually according to the conventional hoisting and clamping apparatus, this applies a heavy burden to the operators at a working site. Accordingly, there is developed a hoisting and clamping apparatus which automatically performs the engagement or releasing of the engagement between the hoisting and clamping apparatus and the heavy object by vertically moving a wire or chain suspended by a crane. For example, such hoisting and clamping apparatus is disclosed in JP-A 6-191786. This hoisting and clamping apparatus comprises double extendible cylinders wherein an inclined plate is fixed inside an upper cylinder and a T-shaped rod protrudes in a lower cylinder. In this structure, the rod is alternately retained by the inclined plate at two parts thereof and a wire of the crane is vertically moved to change the lengths of the double cylinders, thereby alternately performing a clamping or releasing operation.
In this hoisting and clamping apparatus, since the amount of opening of an arm is determined by a rate of expansion of a pantograph and a cam shape, the amount of movement of the arm is not constant because of dispersion of the length of the heavy object to be clamped so that the heavy object cannot be clamped with assurance. Further, although the clamping of the heavy object can be automatically performed, the arm is liable to be transformed when the load of the heavy object is applied to the arm because the heavy object is clamped by the right and left pantographs, and hence such a hoisting and clamping apparatus cannot be used for a long period of time.
The hoisting and clamping apparatus for clamping the heavy object can be moved to an intended spot by a crane. However, there are many cases in practical construction or building sites that the suspended heavy object must be turned in accordance with the site where the heavy object is arranged. For example, in the operation for constructing a side ditch, the axial direction of a block for use in U-shaped ditch (hereinafter referred to as U-shaped ditching block) is conformed to the longitudinal direction of the ditch so as to arrange the U-shaped ditching block. However, in the conventional hoisting and clamping apparatus, the axial direction of the suspended U-shaped ditching block (heavy object) cannot be turned, and hence the suspended U-shaped ditching block is turned by an operator manually so as to conform the direction of the side ditch to that of the U-shaped ditching block. In the conventional hoisting and clamping apparatus, the hoisting by clamping the heavy object and the releasing of the clamping of the heavy object from the hoisting and clamping apparatus are automatically performed by a crane, leading to the requirement of another operator for turning of the axial direction of the heavy object, which does not save time and labor. Further, since another operator is disposed in a position close to the heavy object, there is a likelihood of risk of injuries to persons owing to the falling of the heavy object.
Still further, a specific block for use in revetment and walls of buildings as a transformed concrete block (hereinafter referred to as simply block) has been widely used recently. This block has such a shape that the position where the presser plates of the hoisting and clamping apparatus contact the heavy object and the center of gravity thereof are not accorded with each other in the same axial line, and hence the center of gravity is displaced from the physical center of the block. When such a block is clamped and hoisted by conventional hoisting and clamping apparatus, the clamping position and the center of gravity are not accorded with each other, and hence the block has not been clamped properly.